This invention relates to a free radical polymerization process.
Free radical polymerization of vinyl monomers has been typically initiated using a peroxide or an azo compound as described in Adv. Polym. Sci., 111, 63 (1994) by D. B. Priddy. Peroxides are known to be hazardous materials and are typically diluted for safer handling with inert compounds known as phlegmatizers as described in U.S. Pat. No. 5,347,055, by Priddy et al. Unfortunately, these phlegmatizers can contaminate the final polymer product. Azo initiators are not very efficient and leave potentially toxic residues in the polymer produced.
Diradical initiators have been used previously in free radical polymerizations to produce high molecular weight vinyl polymers as disclosed in Collect Czech. Chem. Commun. 38:1343 by Borsig et al. According to Borsig et al., a polymerization process using diradical initiators produced a higher molecular weight polymethylmethacrylate when compared to a process using monoradical initiators at the same monomer conversion. However, the polymerization rate is reported to be slower in polymerizations using the diradical initiators.
Another free radical polymerization process using cyclic pinacol ethers as diradical initiators has been reported by Crivello et al. in U.S. Pat. No. 4,675,426. However, these initiators also have low polymerization rates when compared to monoradical initiators.
Diradical initiators have also been formed from donor-acceptor substituted cyclopropene as disclosed in Polym. Bull 25:537 and Macromol. 24:2485, by Hall et al. However, the polystyrene produced using this initiator had a bimodal molecular weight distribution containing low molecular weight fractions which may be attributed to self-termination reactions of the diradical.
Peroxides have also been used as diradical or multifunctional initiators. However, it has been found that efficiency decreases with increasing peroxide functional groups in multifunctional peroxide initiators, as disclosed in Advances in Polymer Science, Vol. 111 "Recent Advances in Styrene Polymerization", by D. B. Priddy, pages 105-108.
Therefore, it is desirable to obtain a process using a diradical initiator which does not need to be diluted for safer handling and initiates free radical polymerization with high efficiency and high polymerization rates.